Electrical surges on electrical conductors are produced as a result of lightning strikes, operation of certain electrical equipment, electromagnetic surges, static electricity, induced voltages, and the like. If such electrical surges are severe, they can break down the insulation of the electrical equipment connected to the electrical conductors carrying the electrical surges and thereby damage the electrical equipment. To prevent such damage, it has been known to protect electrical equipment from damaging electrical surges by connecting surge protectors to the electrical conductors connected to the electrical equipment to be protected.
One commonly used surge protector includes a parallel combination of a gas pressurized discharge tube and an air gap connected between an electrical conductor and ground. The gas discharge tube conducts in the presence of an electrical surge to direct the electrical surge to ground. The air gap operates as a backup protection element for the gas discharge tube in case the gas discharge tube is vented. Thus, if the gas discharge tube fails in a vented condition, the backup air gap breaks down in the presence of an overvoltage electrical surge to conduct the electrical surge to ground.
The use of an air gap as a backup to a gas discharge tube, however, presents several problems. For example, while the gas discharge tube is selected to have a breakdown voltage from the tens of volts to the hundreds of volts depending upon the electrical equipment to be protected, an air gap typically has a breakdown voltage on the order of 1,000 to 1,500 volts. This breakdown voltage offered by an air gap is often too high to provide effective protection. Even if the gas discharge tube has not failed, the backup air gap may fire before the gas discharge tube in response to fast rising transients. This operation can also result in the build up of carbon on the electrodes of the backup air gap. When enough carbon builds up, noise and intermittent shorts may be created which can adversely affect the protected electrical equipment. For example, in telecommunication applications, telephone lines can become noisy, and/or can be rendered inoperable due to intermittent shorts. Furthermore, backup air gaps are susceptible to moisture and other contamination between the electrodes thereof. This contamination may not only cause noise, but also may result in premature failure of the surge protector.
Another known surge protector includes a parallel combination of a gas discharge tube and a metal oxide varistor. A metal oxide varistor typically provides a much lower clamping voltage than the gas discharge tube. Such a surge protector can be used for either high voltage or low voltage applications depending upon the proper selection of the gas discharge tube and the metal oxide varistor. In high voltage applications, the metal oxide varistor is normally a high energy metal oxide varistor having a large diameter, a high capacitance, and high leakage currents. In signal and dataline applications, for example, the clamping voltage of the metal oxide varistor is typically lower than the breakdown voltage of the gas discharge tube. In the protection of low voltage telecommunication equipment used in telephone subscriber stations and in central offices, the surge protector must have low capacitance and high insulation resistance so that the surge protector is transparent to the telecommunication equipment. For example, electrical specifications for telecommunication equipment typically require that the surge protector has a capacitance below 30 pF per line. Compared to this capacitance, conventional hybrid or solid state surge protectors have capacitance values exceeding 30 pF and may exceed several hundred picofarads.
Thus, a very small diameter metal oxide varistor must be chosen in order to present a low capacitance to the protected electrical equipment, particularly telecommunication equipment. At the same time, the metal oxide varistor and the gas discharge tube must ideally be matched so that the clamping voltage of the metal oxide varistor is just above the upper tolerance of the breakdown voltage of the gas discharge tube. This matching ensures that the gas discharge tube is the primary surge protection element in the surge protector and that the metal oxide varistor provides backup protection in case the gas discharge tube fails to properly operate.
Surge protectors, particularly those used in telecommunication systems, must be capable of offering protection in spite of a power cross or a failure of any of the protective elements. Power cross, particularly in the telecommunication arts, occurs when live alternating current power distribution cables come in direct contact with telephone wires causing high voltage alternating current power to be conducted through low voltage local telephone circuits. This high voltage alternating current power can heat and overstress the surge protective devices in the surge protector and cause a thermal overload condition. If adequately designed, the surge protector will provide a "failsafe" condition by shorting the affected line to ground.